Refrigerant Distribution
The distribution system is selected based upon the type of defrost for that particular system. For each set of liquid/suction lines a distribution system must be selected.
Liquid solenoids are recommended to be installed at the evaporator on all systems, particularly systems with long line runs. The solenoid will prevent continued feed to the evaporator through the expansion valve when it is not in operation. A solenoid is mentioned in each of the refrigerant distribution analysis, and are shipped loose to be installed at the evaporators.
Heatcraft offers three types of defrost: Off cycle defrost, Electric defrost and the Priority I hot gas defrost system for Racks. The type of defrost is generally a matter of either contractor or owner preference. Typical operation is as follows:
Off-Cycle
The
Electric Defrost
The electric defrost system consists of liquid and suction line ball valves for circuit isolation, liquid solenoid, evaporator heater contactor, heater fusing, evaporator fan motor contactor and fuses if three phase fans are used, and defrost controller.
Defrost is initiated by the controller. The liquid solenoid closes, the evaporator fan contactor opens stopping the fans, and the defrost heater contactor is energized.
When the defrost heaters warm the coil to a predetermined level an adjustable defrost termination device within the evaporator signals the defrost controller to end the defrost period. A fan delay is provided at the end of each defrost cycle to allow the evaporator to cool before the fans start. This also prevents warm air and condensation from being discharged from the unit. The liquid solenoid opens putting the system back into refrigeration.
Priority I Hot Gas Defrost
For Racks Only
The Priority I Hot Gas Defrost system consists of liquid and suction line ball valves for circuit isolation, liquid line solenoid with by pass check valve, suction solenoid valve, hot gas solenoid valve, liquid drain solenoid valve, liquid drain manifold, and defrost controller.
Defrost is initiated by the defrost controller closing the liquid solenoid and suction solenoid. The hot gas and liquid drain solenoids open (Unlike typical systems wherein the condensed liquid from the defrosting evaporator is returned into the liquid manifold, the Priority I design returns the liquid to the condenser through a liquid drain manifold).
Hot discharge gas is injected into the suction line at the parallel rack and flows to the evaporator being defrosted. The
Installation and Operations Manual
discharge gas will condense into liquid as it flows through the cold evaporator.
The liquid exits the coil at the distributor
The pressure in the condenser is controlled to be below the returning liquid pressure by a discharge gas regulator valve. The returning liquid pressure helps in driving refrigerate from the condenser to the receiver to maintain liquid refrigerant flows to the refrigerating evaporators. The Priority I system requires that no more than 20% of the evaporators defrost at one time.
The discharge gas regulator valve (DDGR) is normally set to maintain approximately 25 psig differential pressure. The next part of the Priority I system consists of a small capacity control system located at the compressor rack. The discharge gas bypass regulator valve should be set to maintain the normal suction pressure during normal refrigeration. There is a desuperheating TXV mounted to prevent overheating the suction line. The expansion valve should not require an adjustment as it is preset to maintain 20°F superheat. See page 31 of this manual for more information on the adjustment of the discharge gas bypass regulator valve.
Head Pressure Control System
Almost all refrigeration systems require some form of year round head pressure control. This is due to the fixed amount of condenser surface which has been selected for summer conditions. During the winter, the condenser is oversized for the system and low head pressure will result. This will cause erratic operation of the system.
The following method of head pressure control is considered the most effective means and has the advantage of performing well at low outside ambient temperatures. The disadvantage is the fact that a relatively large quantity refrigerant must be used to flood the condenser and sufficient receiver storage must be provided during summer operation.
Head pressure control system consists of a condenser drain line valve and a discharge bypass valve. In order to maintain moderate head pressure the condenser drain valve senses condensing pressure. As condensing pressure falls in response to lower ambient temperatures, the drain valve will begin to restrict flow of liquid from the condenser filling condenser tubes with liquid refrigerant. This results in decreased surface area causing the discharge pressure to rise.
When pressure reaches the midpoint setting the valve begins to open allowing liquid to flow to the receiver. Simultaneously the discharge bypass valve installed in a line between the discharge manifold and the receiver maintains minimum receiver pressure to insure liquid flow.
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